Detection of fluid leaks, such as natural gas leaks from pipelines, is well known in the art. It is difficult to detect failures of gas and oil pipelines, because the gas or oil pipeline is typically buried beneath ground level. When failures do occur, they are manifest by the leakage of the pipeline contents, where the leaking material produces a characteristic trace or signal which may be measured. Because fluids can escape from a pipeline and travel through subterranean earth to the earth's surface and then into the atmosphere, the atmosphere can be monitored for the trace fluids.
Differential absorption LIDAR (DIAL) systems may be used to remotely measure the chemical composition of fluids in the atmosphere. DIAL systems may be provided on an airborne or a ground-based platform. In a DIAL system, at least two lasers having different wavelengths are transmitted to a survey area for trace fluid detection from a remote location. Although a DIAL system may use two or more lasers, it is understood that, in general, a DIAL system uses at least two different wavelengths for trace fluid detection. It is contemplated that a single laser may be used in a scheme similar to frequency hopping to select different wavelengths or may be tuned to different wavelengths. The wavelength of one of the lasers, referred to as the on-line laser, is typically selected to coincide with a strong absorption feature of the fluid to be detected. The wavelength of another of the lasers, referred to as the off-line laser, is typically selected such that it is not absorbed by the target fluid. The transmitted laser beams may be reflected, scattered and/or absorbed before being received by the DIAL system. If the target fluid is present, a portion of the transmitted on-line laser energy may be absorbed by the target fluid and the received energy may be different from the received off-line laser energy. If the target fluid is not present, both received laser energies may be approximately equal. A difference in received energies may be used to estimate a concentration path length (CPL) of the target fluid used for estimating a concentration of the target fluid.
Natural gas, for example, characteristically contains a mixture of methane, ethane, and small amounts of other gases. Oil pipelines also contain significant concentrations of volatile dissolved gas compounds, including methane, ethane, and propane. Gas may also be generated by the decomposition of organic matter, henceforth, referred to as swamp gas, and may only contain methane. Measurement of the expected components and a confirmation of the appropriate concentration ratio between these components can thus be used to directly establish the presence of a pipeline leak. It is contemplated that swamp gas may also be distinguished from a target fluid by observing a shape, spatially, of the emitted plume. Swamp gas may have a different shape as compared with a target fluid. In general, it is highly desirable for any fluid detection method to be able to distinguish between released gases resulting from a failure in a pipeline or a holding container versus emanating swamp gases, thus avoiding false alarms.
It will be appreciated that in many DIAL systems, the on-line returns are typically not much higher in energy than the background noise. This low signal-to-noise ratio (SNR), when the target fluid is present in the survey area, results in ambiguities or difficulties in detecting the on-line returns. Variation in spectral surface reflectivity typically causes a corresponding variation in the on-line and off-line returns. This variation is made more pronounced in the detection algorithm if the variation in the off-line is in an opposite direction as the on-line (e.g. the off-line reflectivity is higher and the on line reflectivity is lower). It is also made more pronounced if there is misalignment of the on-line and off-line beams (partially overlapping beams).
Low surface cover reflectivity results in low off-line and on-line returns whereas high surface cover reflectivity results in high returns. When the returned signal is low relative to noise, electrical noise may dominate and cause a low SNR and large CPL variance, but the opposite is also true. When the returns are high relative to noise, the signal dominates, leading to a high SNR and low CPL variance. Because the surface cover reflectivity varies from point to point and from region to region, so do the off-line and on-line returns and thus the SNR.
In practice, DIAL systems are calibrated. However, it may be difficult to correct for reflectivity variations due to the type of surface cover in many situations. If the surface cover reflectivity variations are not properly corrected, significant errors in CPL estimates of the target fluid may result, leading to false identification of target fluid plumes (or lack of plumes).
U.S. Pat. No. 6,822,742, issued on Nov. 23, 2004 to Kalayeh et al., entitled SYSTEM AND METHOD FOR REMOTE QUANTITATIVE DETECTION OF FLUID LEAKS FROM A NATURAL GAS OR OIL PIPELINE, provides a system for remote quantitative detection of fluid leaks from a natural gas or oil pipeline by use of an airborne platform. The contents of the above referenced application are incorporated herein by reference in their entirety.